Amplification type solid-state image pickup device driving method

ABSTRACT

A plurality of pixel groups X(n) each comprising a plurality of pixels are set, and switched capacitor amplification parts are provided in correspondence to the pixel groups, respectively. Each of the switched capacitor amplification parts has a charge detection node to which output terminals of the transfer transistors of a corresponding pixel group X(n) are connected in common, an amplification part, a reset transistor, a first capacitance element, and a select transistor. A load part common to the switched capacitor amplification parts is provided. The load part is combined with the amplification parts of the switched capacitor amplification parts to constitute inverting amplifiers, respectively. By means of the above constitution, it is capable of obtaining a noise-reduced, high-quality image and which allows transistor count per pixel to be cut, thus allowing the pixel size to be reduced.

This nonprovisional application claims priority under 35 U.S.C. §119(a)on Patent Application No. 2004-019782 filed in Japan on Jan. 28, 2004,the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to an amplification type solid-state imagepickup device and a driving method therefor. More specifically, theinvention relates to an amplification type solid-state image pickupdevice, as well as a driving method therefor, which includes a pluralityof pixels each having a photoelectric conversion element and a transfertransistor for transferring signal charge of the photoelectricconversion element, where signals derived from the individual pixels areamplified and outputted to a signal line in common to the pixels.

Generally, there has been wide-spreading an amplification typesolid-state image pickup device which has a pixel part provided with anamplification function and a scanning circuit disposed around the pixelsection, where pixel data is read from the pixel section by the scanningcircuit. In particular, there has been known an APS (Active PixelSensor) type image sensor formed of CMOS (Complementary Metal OxideSemiconductor) which is advantageous for integration of the pixel partwith peripheral drive circuit and signal processing circuit.

The APS type image sensor normally includes, in one pixel, aphotoelectric conversion part, an amplification part, and a pixelselection part and a reset part. Therefore, for makeup of the APS typeimage sensor, normally, three to four MOS transistors (Tr) are used inaddition to the photoelectric conversion part formed of photodiodes(PD).

Providing three to four MOS transistors per pixel as shown above wouldbecome a constraint on reduction of the pixel size. Therefore, lately,there has been proposed a technique for reducing the number oftransistors per pixel as shown in FIG. 12 (see, e.g., JP H09-46596A).

The amplification type solid-state image pickup device shown in FIG. 12includes a plurality of pixels each made up of a photodiode 101 and atransfer transistor 102, and further includes a charge detection node108 common to a plurality of pixels arrayed in a column direction, areset transistor 131, an amplification transistor 132 and a selecttransistor 133. Between a vertical signal line 135 and the ground isinterposed a constant-current load transistor 134. All the transfertransistors 102, 131, 132 and 133 are n-channel transistors, and turnedON and OFF depending on High or Low of gate driving signals,respectively.

As shown in FIG. 13, in a period T1, a gate driving signal φR(m) to beapplied to the reset transistor 131 goes High level, causing thepotential under the gate to become deeper, where there occurs a chargemove from the charge detection node 108 to the drain side of the resettransistor 131, causing the voltage of the charge detection node 108 tobe reset to the power supply voltage V_(DD) (reset level).

In the next period T2, the gate driving signal φR(m) goes Low level,causing the reset transistor 131 to turn OFF. Meanwhile, since the gatedriving signal φS(m) applied to the select transistor 133 goes Highlevel, the reset level is read to the vertical signal line 135 via theamplification transistor 132 and the ON-state select transistor 133. Inthis case, the amplification transistor 132 and the constant-currentload transistor 134 constitute a source follower circuit.

In the next period T3, the gate driving signal φS(m) goes Low level,causing the select transistor 133 to turn OFF. Meanwhile, since a gatedriving signal φT(m, 1) applied to the transfer transistor 102 of the1st line of the m-th row goes High level, the potential under the gatebecomes deeper, so that the signal charge (electrons) stored in thephotodiode 101 the 1st line of the m-th row is transferred to the chargedetection node 108.

In the next period T4, the gate driving signal φT(m, 1) goes Low level,causing the transfer transistor 102 of the 1st line of the m-th row toturn OFF, while the charge detection node 108 is held at the voltage ofthe signal charge transfer. Meanwhile, because the gate driving signalφS(m) goes High level, a signal level of the m-th row is read to thevertical signal line 135 via the amplification transistor 132 and theON-state select transistor 133.

After one horizontal scan period (1H period), for the pixel of the 2ndline of the m-th row, signal charge derived from the photodiode 101 ofthe 2nd line of the m-th row is led to the reset transistor 131, theamplification transistor 132 and the select transistor 133 via thetransfer transistor 102 of the 2nd line of the m-th row. Then,operations similar to those of the foregoing T1 to T4 are performed.

In this proposed amplification type solid-state image pickup device, forexample, an assumption that one common part (i.e., charge detection node108, reset transistor 131, amplification transistor 132 and selecttransistor 133) is given for each two pixels is equivalent to 2.5transistors per pixel. Also, one common part provided for each fourpixels is equivalent to 1.75 transistors per pixel. Therefore, thenumber of transistors per pixel is reduced, as compared with a generalAPS image sensor comprising three to four MOS transistors per pixel.

However, with the technique of JP H09-46596A, there would arise problemsas shown below. That is, given that the capacitance of the common chargedetection node 108 is C_(FD), a charge-voltage conversion efficiency nat which signal charge Qsig derived from the photodiode 101 is convertedto a voltage signal Vsig isη=G·Vsig/Qsig=G/C _(FD)where G is the gain of the source follower circuit made up of theamplification transistor 132 and the constant-current load transistor134, being generally smaller than 1.

As apparent from this equation, the capacitance C_(FD) needs to bereduced in order to increase the charge-voltage conversion efficiency η.The capacitance C_(FD) of the common charge detection node 108 is atotal sum of drain-side junction capacitances of a plurality of transfertransistors 102 connected to the charge detection node 108, a gatecapacitance of the amplification transistor 132, and a node-sidejunction capacitance of the reset transistor 131. Therefore, thecapacitance C_(FD) increases with increasing number of pixels(photodiodes 101 and transfer transistors 102) connected to a commoncharge detection node 108, which leads to a problem that thecharge-voltage conversion efficiency η decreases.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide anamplification type solid-state image pickup device, as well as a drivingmethod therefor, which is capable of obtaining noise-reducedhigh-quality images and reducing the transistor count per pixel tominiaturize the pixel size.

In order to achieve the above object, according to the presentinvention, there is provided an amplification type solid-state imagepickup device which comprises a plurality of pixels each having aphotoelectric conversion element and a transfer transistor fortransferring signal charge of the photoelectric conversion element,wherein signals derived from the respective pixels are amplified andoutputted to a signal line common to the pixels, and wherein:

a plurality of pixel groups each comprising a plurality of pixels areset;

switched capacitor amplification parts are provided in correspondence tothe pixel groups, respectively;

each of the switched capacitor amplification parts has a chargedetection node to which output terminals of the transfer transistors ofa corresponding one of the pixel groups are connected in common, anamplification part to which a signal on the charge detection node is tobe inputted, a reset transistor connected between the charge detectionnode and an output terminal of the amplification part, a firstcapacitance element connected between the charge detection node and theoutput terminal of the amplification part, and a select transistorconnected between the output terminal of the amplification part and thesignal line; and

a load part common to the switched capacitor amplification parts isconnected between the signal line and power supply of a specifiedvoltage so as to be combined with the amplification parts of theswitched capacitor amplification parts to make up inverting amplifiers,respectively.

Herein, the term “connected” refers to being electrically connected.

In the amplification type solid-state image pickup device of thisinvention, the transistor and the switched capacitor amplification partare controlled properly so that the operation of reading a signal fromthe photoelectric conversion element via the transfer transistor foreach pixel in each pixel group is iterated, by which signals derivedfrom the respective pixels can be amplified and outputted to the signalline (which will be detailed later). According to the amplification typesolid-state image pickup device of the invention, since the switchedcapacitor amplification part (including the amplification part) iscommon to a plurality of pixels included in each pixel group, the numberof transistors per pixel can be reduced. Moreover, since the load partis provided in common to the switched capacitor amplification parts, thestructure of the switched capacitor amplification part is simplified, ascompared with the case where the load part is provided for each switchedcapacitor amplification part. Accordingly, the number of transistors perpixel can be further reduced. Further, since the amplification part isof the switched capacitor type, it becomes possible to effectivelyreduce the capacitance of the charge detection node, so that thecharge-voltage conversion efficiency can be enhanced. Thus, evenless-noise, higher-quality images can be obtained.

In one embodiment of the amplification type solid-state image pickupdevice, the photoelectric conversion element is a buried photodiode.

It is noted here that the terms “buried photodiode” refer to aphotodiode in which a pn junction is formed in a semiconductor separatefrom the surface of the semiconductor substrate so that dark currentsgenerated at the surface of photodetection part are not read out.

In this amplification type solid-state image pickup device of oneembodiment, since the photoelectric conversion element is a buriedphotodiode, signal charge stored in the photodiode is transferredwithout being lost. Therefore, it becomes achievable to reduce noise andobtain higher-quality images.

In one embodiment of the amplification type solid-state image pickupdevice, the load part comprises a transistor serving as aconstant-current load.

In this amplification type solid-state image pickup device of oneembodiment, the amplification part and the load part constitute aconstant-current load type inverting amplifier, so that theamplification type solid-state image pickup device operates stably.

In one embodiment of the amplification type solid-state image pickupdevice, the load part comprises a diffusion layer in which impuritiesare diffused in a semiconductor.

In this amplification type solid-state image pickup device of oneembodiment, since the load part comprises a diffusion layer in whichimpurities are diffused in a semiconductor, the load part formed withrelatively high resistance. In this case also, the amplification partand the load part constitute an inverting amplifier, so that theamplification type solid-state image pickup device operates stably.

Preferably, a grounding terminal of each switched capacitoramplification part consists of a light-shielding interconnect patterncommon to all the pixels.

In one embodiment of the amplification type solid-state image pickupdevice, the amplification part and the load part constitute a cascodetype inverting amplifier.

In this amplification type solid-state image pickup device of oneembodiment, since the amplification part and the load part constitute acascode type inverting amplifier, the gain becomes larger, as comparedwith the constant-current load source-grounded type. Therefore, thecharge-voltage conversion efficiency can be further enhanced, and evenless-noise, higher-quality images can be obtained.

In one embodiment, the amplification type solid-state image pickupdevice further comprises a control part for controlling the transfertransistor and the switched capacitor amplification part so that anoperation of reading a signal from the photoelectric conversion elementvia the transfer transistor is iterated for each of the pixels in eachof the pixel groups.

In this amplification type solid-state image pickup device of oneembodiment, the control part controls the transfer transistor and theswitched capacitor amplification part so that an operation of reading asignal from the photoelectric conversion element via the transfertransistor is iterated for each of the pixels in each of the pixelgroups. Thus, signals derived from the respective pixels can beamplified and outputted to the signal line.

In another aspect of the present invention, there is provided anamplification type solid-state image pickup device driving method fordriving the amplification type solid-state image pickup device, whichcomprises:

a first step of turning on the reset transistor of the switchedcapacitor amplification part to perform a reset operation;

a second step of, after the first step, turning on the select transistorand turning off the reset transistor to perform a read operation of onlya reset level;

a third step of, after the second step, turning on the transfertransistor of the pixel to perform charge transfer from the pixel to theswitched capacitor amplification part; and

a fourth step of, after the third step, turning off the transfertransistor and turning on the select transistor to perform a readoperation of a signal level, wherein

operations of the first step to the fourth step are iterated for each ofthe pixels in each of the pixel groups.

In one embodiment, the amplification type solid-state image pickupdevice further comprises:

a second capacitance element whose one terminal is connected to thecharge detection node; and

first boosting means connected to the other terminal of the secondcapacitance element and serving for deepening potential of the chargedetection node by capacitive coupling via the second capacitanceelement.

In this amplification type solid-state image pickup device of oneembodiment, by the first boosting means, potential of the chargedetection node is deepened by capacitive coupling via the secondcapacitance element, by which charge transfer from the photoelectricconversion element to the charge detection node is accelerated.Accordingly, noise can be further reduced, and higher-quality images canbe obtained.

In another aspect of the invention, there is provided an amplificationtype solid-state image pickup device driving method for driving theamplification type solid-state image pickup device, which comprises:

a first step of turning on the reset transistor of the switchedcapacitor amplification part to perform a reset operation;

a second step of, after the first step, turning on the select transistorand turning off the reset transistor to perform a read operation of onlya reset level;

a third step of, after the second step, turning on the transfertransistor of the pixel, in a state that potential of the chargedetection node has been deepened by the capacitive coupling via thesecond capacitance element by the first boosting means, to performcharge transfer from the pixel to the switched capacitor amplificationpart; and

a fourth step of, after the third step, turning off the transfertransistor and turning on the select transistor to perform a readoperation of a signal level, wherein

operations of the first step to the fourth step are iterated for each ofthe pixels in each of the pixel groups, whereby the signal is read outfrom each of the photoelectric conversion elements of the pixel groups.

In one embodiment of the amplification type solid-state image pickupdevice, terminals of the reset transistor and the first capacitanceelement included in the switched capacitor amplification part other thanand opposite to their terminals connected to the charge detection nodeare connected to the signal line instead of the output terminal of theamplification part, and wherein

the amplification type solid-state image pickup device further comprisesa second boosting means connected to the signal line and serving fordeepening potential of the charge detection node by capacitive couplingvia the first capacitance element.

In this amplification type solid-state image pickup device of oneembodiment, by the second boosting means, potential of the chargedetection node is deepened by capacitive coupling via the firstcapacitance element, by which charge transfer from the photoelectricconversion element to the charge detection node is accelerated.Accordingly, noise can be further reduced, and higher-quality images canbe obtained.

In another aspect of the invention, there is provided an amplificationtype solid-state image pickup device driving method for driving theamplification type solid-state image pickup device, which comprises:

a first step of turning on the reset transistor of the switchedcapacitor amplification part to perform a reset operation;

a second step of, after the first step, turning on the select transistorand turning off the reset transistor to perform a read operation of onlya reset level;

a third step of, after the second step, turning on the transfertransistor of the pixel, in a state that potential of the chargedetection node has been deepened by the capacitive coupling via thefirst capacitance element by the second boosting means, to performcharge transfer from the pixel to the switched capacitor amplificationpart; and

a fourth step of, after the third step, turning off the transfertransistor and turning on the select transistor to perform a readoperation of a signal level, wherein

operations of the first step to the fourth step are iterated for each ofthe pixels in each of the pixel groups.

As shown above, the present invention is greatly useful for theformation of small-size, high-performance image sensors.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention, and wherein:

FIG. 1 is a view showing the construction of a two-dimensionalamplification type solid-state image pickup device according to anembodiment of the present invention;

FIG. 2 is a timing chart showing operation timing of the two-dimensionalamplification type solid-state image pickup device of FIG. 1;

FIG. 3 is a view showing circuit construction of a constant-current loadtype source-grounded inverting amplifier made up of the elements of FIG.1;

FIG. 4 is a view showing characteristics of the inverting amplifier ofFIG. 3;

FIG. 5 is a view showing a modification of the two-dimensionalamplification type solid-state image pickup device of FIG. 1;

FIG. 6 is a view showing circuit construction of a cascode typeinverting amplifier made up of elements in FIG. 5;

FIG. 7 is a view showing characteristics of the inverting amplifier ofFIG. 6;

FIG. 8 is a view showing another modification of the two-dimensionalamplification type solid-state image pickup device of FIG. 1;

FIG. 9 is a timing chart showing operation timing of the two-dimensionalamplification type solid-state image pickup device of FIG. 8;

FIG. 10 is a view showing another modification of the two-dimensionalamplification type solid-state image pickup device of FIG. 1;

FIG. 11 is a timing chart showing operation timing of thetwo-dimensional amplification type solid-state image pickup device ofFIG. 10;

FIG. 12 is a view showing the structure of a two-dimensionalamplification type solid-state image pickup device according to theprior art.

FIG. 13 is a timing chart showing operation timing of thetwo-dimensional amplification type solid-state image pickup device ofFIG. 12;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinbelow, an amplification type solid-state image pickup device and adriving method therefor of the present invention are described in detailby reference to an embodiment shown in the accompanying drawings.

FIG. 1 is a circuit diagram showing part of a two-dimensionalamplification type solid-state image pickup device according to anembodiment of the present invention. The two-dimensional amplificationtype solid-state image pickup device includes a plurality of pixels 10each having a buried photodiode 1 as a photoelectric conversion elementand a transfer transistor 2 for transferring signal charge of thephotodiode 1. Those pixels 10 are arrayed two-dimensionally in a matrix,and divided into pixel groups X(n−1), X(n), X(n+1), . . . (hereinafter,referred to generically as pixel groups X when appropriate) each ofwhich is composed of k pixels (where k≧2) in each column. It is notedthat FIG. 1 shows in detail the n-th pixel group X(n) in one column (the1st column in this example) alone (where n≧2) for simplicity's sake.Such pixel groups X are set in pluralities in row and column directions,respectively.

Each pixel group X is defined by forming a charge detection node 8 towhich output terminals of the transfer transistors 2 belonging to thepixel group are connected in common. The capacitance of the chargedetection node 8 is represented by capacitance C_(FD). It is noted thatcharge detection nodes 8 are isolated from each other between pixelgroups X.

Reference characters T(n, 1), T(n, 2), . . . , T(n, k) denote gatedriving signal lines for the transfer transistors 2 belonging to thepixel group X(n), and gate driving signals φT(n, 1), XT(n, 2), . . . ,φT(n, k) are to be applied thereto, respectively, by a control section90.

Also, switched capacitor amplification parts 20 are provided incorrespondence to the pixel groups X, respectively. Each switchedcapacitor amplification part 20 is interposed between a correspondingpixel group X and a vertical signal line 11 common to a column to whichthe pixel group X belongs.

The switched capacitor amplification part 20 has a charge detection node8 to which output terminals of the individual transfer transistors 2 ofa corresponding pixel group X (hereinafter, focused on X(n) fordescription) are connected in common, an amplification transistor 3 asan amplification part to which a signal on the charge detection node 8is to be inputted, a reset transistor 6 connected between the chargedetection node 8 and an output terminal 30 of the amplificationtransistor 3, a capacitor 7 (whose capacitance is represented by Cin) asa first capacitance element connected between the charge detection node8 and the output terminal 30 of the amplification transistor 3, and aselect transistor 5 connected between the output terminal 30 of theamplification transistor 3 and the vertical signal line 11.

Reference character R(n) denotes a gate driving signal line for thereset transistor 6, and a gate driving signal φR(n) is to be appliedthereto by the control section 90.

Reference character S(n) denotes a gate driving signal line for theselect transistor 5, and a gate driving signal φS(n) is to be appliedthereto by the control section 90.

A load part 21 is interposed and connected between the vertical signalline 11 and a power supply of the voltage VDD. In the load part 21, aconstant-current load transistor 4 serves as a load common toamplification transistors 3 of the individual switched capacitoramplification parts 20 corresponding to one column (the 1st column inthis case). In this case, the load part 21 is implemented by theconstant-current load transistor 4.

For operation, a constant bias voltage Bias_p for giving a flow of aconstant current through the constant-current load transistor 4 isapplied to its gate. Thus, the amplification transistor 3 and theconstant-current load transistor 4 constitute a constant-current loadtype source-grounded inverting amplifier. Then, the amplificationtransistor 3, the constant-current load transistor 4 and the capacitor 7constitute a switched capacitance amplifier. In addition, the load part21 does not have to be a constant-current load transistor 4, and thesame objective can be achieved even if the load part 21 is formed of adiffusion layer or the like with high resistance. Also, the groundingterminals of the amplification transistors can be interconnected bymeans of a light-shielding metal common to the pixels.

Reference character C(n) denotes a boosting signal line, and a boostingsignal φC(n) is to be applied thereto by the control section 90. Betweenthis boosting signal line C(n) and the charge detection node 8 isconnected a capacitor 9 (whose capacitance is represented by capacitanceCup) as a second capacitance element. In operation, the boosting signalφC(n) is raised to a high voltage at a specified timing by the controlsection 90. Thus, the boosting signal line C(n) and the control section90 serve as a first boosting means, so that the potential of the chargedetection node 8 can be deepened by capacitive coupling via thecapacitor 9.

FIG. 2 shows operation timing of the two-dimensional amplification typesolid-state image pickup device under the control by the control section90 by focusing on the pixel group X(n) and the switched capacitoramplification part 20 corresponding thereto.

In a period T1, a gate driving signal φS(n) applied to the selecttransistor 5 goes High level and the gate driving signal φR(n) appliedto the reset transistor 6 goes High level, where those transistors 5, 6go ON state. As a result, by the action of the constant-current loadtype source-grounded inverting amplifier, which is constituted of theamplification transistor 3 and the constant-current load transistor 4,the voltage of the charge detection node 8 is reset to a constantvoltage V0 (reset level).

The reset level Vo is defined as follows. That is, when the transistors5, 6 are turned ON and short-circuited as described above, the circuitof the constant-current load type source-grounded inverting amplifier,which is constituted of the amplification transistor 3 and theconstant-current load transistor 4, is represented as shown in FIG. 3.It is assumed here that an input of the inverting amplifier is Vin andan output thereof is Vout, and further that a characteristic of theinverting amplifier is F1 as shown in FIG. 4. When the transistors 5, 6are turned ON and short-circuited, it follows that Vout=Vin, so that thereset level Vo is defined as an intersection point between thecharacteristic F1 and the straight line of Vout=Vin.

In the next period T2, the gate driving signal φR(n) goes Low level,causing the reset transistor 6 to turn OFF. Meanwhile, the gate drivingsignal φS(n) remains as it is at High level, and the select transistor 5has been in the ON state. Therefore, an output resulting from invertingand amplifying the voltage of the charge detection node 8, i.e. thereset level Vo, is read to the vertical signal line 11 via the ON-stateselect transistor 5.

In the next period T3, the gate driving signal 4S(n) goes Low level,causing the select transistor 5 to turn OFF. At this point, the gatedriving signal φT(n, 1) goes High level, and the transfer transistor 2of the 1st pixel 10 in the pixel group X(n) turns ON. Thus, the signalcharge stored in the photodiode 1 of the 1st pixel 10 is transferred tothe charge detection node 8 through the ON-state transfer transistor 2.Further, in synchronization with the gate driving signal φT(n, 1), theboosting signal φC(n) goes High level. Thus, the potential of the chargedetection node 8 is deepened by capacitive coupling via the capacitor 9(with its capacitance Cup). Accordingly, charge transfer from thephotodiode 1 to the charge detection node 8 is accelerated.

It is noted that potential change of the charge detection node 8 isequivalent to the one that results from distributing a voltage increaseof the boosting signal line C(n) by the capacitance Cup of the capacitor9 and the capacitance C_(FD) of the charge detection node 8.

In the next period T4, the gate driving signal φT(n, 1) goes Low level,causing the transfer transistor 2 to turn OFF. Also, since the boostingsignal φC(n) goes Low level, the potential change of the chargedetection node 8 by the capacitive coupling via the capacitor 9 iscanceled. As a result, a voltage (signal level) shifted from the resetlevel (voltage V0) in the period T2 by an extent due to the signalcharge transfer in the period T3 is held at the charge detection node 8.This signal level is amplified by the constant-current load typesource-grounded inverting amplifier, which is constituted of theamplification transistor 3 and the constant-current load transistor 4,and read to the vertical signal line 11 through the ON-state selecttransistor 5 (because the gate driving signal φS(n) has been turned ON).

Then, under the control by the control section 90, extracting adifference signal between the reset level of the period T2 and thesignal level of the period T4 read to the vertical signal line 11 makesit possible to obtain an effective signal by the charge generated fromthe light incident on the 1st pixel 10 in the pixel group X(n).

After one horizontal scan period (1 H period), for the 2nd pixel in thepixel group X(n), operations similar to those of the foregoing periodsT1 to T4 are performed.

In this way, by iterating the operations of the periods T1 to T4 foreach pixel 10 in each pixel group X, a signal derived from each pixel 10can be amplified and outputted to the vertical signal line 11 fromcolumn to column. The signals read to the vertical signal line 11 areoutputted in succession through an unshown horizontal signal lineprovided in common to the vertical signal line.

Now, given a charge amount Qsig transferred from the photodiode 1 and again A of the constant-current load type source-grounded invertingamplifier, an effective signal to be read isVsig=A·Qsig/[C_(FD)+Cup+(1+A)Cin]  (1)where the gain A of the constant-current load type source-groundedinverting amplifier isA=gm ·(r _(on) //r _(op))  (2)In Equation (2), gm is the transconductance of the amplificationtransistor 3, r_(on) is the output resistance of the amplificationtransistor 3, and r_(op) is the output resistance of theconstant-current load transistor 4.

In particular, on condition that the amplification gain A is quitelarge, it follows from Equation (1) thatVsig˜Qsig/Cin  (3)Therefore, the charge-voltage conversion efficiency η isη=Vsig/Qsig=1/Cin  (4)As can be understood from Equation (4), when the amplification gain A isquite large, there are substantially almost no effects of thecapacitance C_(FD) of the charge detection node 8 on outputted signals.Therefore, even if the number of pixels connected in the columndirection is increased with C_(FD) increased, no decreases in thecharge-voltage conversion efficiency η occur.

Still, according to this two-dimensional amplification type solid-stateimage pickup device, since the switched capacitor amplification part 20(including the amplification part) is common to a plurality (k in thiscase) of pixels 10 included in each pixel group X, the number oftransistors per pixel can be reduced. Moreover, since the load part 21is provided in common to the switched capacitor amplification parts 20of one column, the structure of the switched capacitor amplificationpart 20 is simplified, as compared with the case where the load part 21is provided for each switched capacitor amplification part. Accordingly,the number of transistors per pixel can be further reduced.

Further, since the photodiode 1 is a buried type photodiode, signalcharge stored in the photodiode 1 is transferred without being lost.Therefore, noise reduction can be achieved so that a high-quality imagebecomes obtainable. Still, since the amplification part 20 is of theswitched capacitor type, it becomes possible to effectively reduce thecapacitance C_(FD) of the charge detection node 8, so that thecharge-voltage conversion efficiency η can be enhanced. Thus, evenless-noise, higher-quality images can be obtained.

The terms “buried photodiode” refer to a photodiode in which a pnjunction is formed in a semiconductor separate from the surface of thesemiconductor substrate so that dark currents generated at the surfaceof photodetection part are not read out. At the surface of thephotodetection part of general photodiodes, a Si—SiO₂ interface ispresent and crystal defects tend to occur. Applying a readout voltage tothis part causes electric charge to be generated even in a dark state,making a noise source. For suppression of this, for example, an n-typeregion is diffused and formed on the surface of a p-type semiconductorsubstrate, and further p⁺-type region is formed so as to cover then-type region, by which the n-type region is formed as the “buriedtype.” Then, even with the readout voltage applied, since the darkcurrent generated at the surface is not read out, noise due to the darkcurrent can be reduced.

FIG. 5 shows a modification of the two-dimensional amplification typesolid-state image pickup device. This two-dimensional amplification typesolid-state image pickup device is equivalent to that of FIG. 1 in whichthe amplification part within each switched capacitor amplification part20B as well as a load part 21B combined therewith are structurallymodified. The rest of the constituent members and operation timing arethe same as those of FIG. 1. It is noted that, in FIG. 5, the sameconstituent members as those of FIG. 1 are designated by like referencenumerals and their description is omitted.

The two-dimensional amplification type solid-state image pickup deviceof FIG. 5 includes, as the amplification part, an amplificationtransistor 31 and an n-channel cascode transistor 32 connected inseries. A signal on the charge detection node 8 is inputted to the gateof the amplification transistor 31. A constant bias voltage Bias_n1 forturning ON the transistor upon operation is inputted to the gate of then-channel cascode transistor 32.

The load part 21B is comprised of a constant-current load transistor 41and a p-channel cascode transistor 42 connected in series. To gates ofthe constant-current load transistor 41 and the p-channel cascodetransistor 42, upon operation, are applied constant bias voltagesBias_p1, Bias_p2 for making those transistors serve as constant-currentloads, respectively.

The circuit of the cascode type inverting amplifier, which is formedfrom a combination of the amplification part within the switchedcapacitor amplification part 20B and the load part 21B, is representedas shown in FIG. 6. It is assumed here that an input of the invertingamplifier is Vin and an output thereof is Vout, and further that acharacteristic of the inverting amplifier is F1 as shown in FIG. 7. Whenthe select transistor 5 and the reset transistor 6 within the switchedcapacitor amplification part 20B are turned ON and short-circuited, itfollows that Vout=Vin, so that the reset level Vo is defined as anintersection point between the characteristic F2 and the straight lineof Vout=Vin.

The gain A of the cascode type inverting amplifier isA=gm ₁[(gm ₂ ·r _(on1) ·r _(on2))//(gm ₃ ·r _(op3) r _(op4))]  (5)In Equation (5), gm₁, gm₂ and gm₃ are transconductances of theamplification transistor 31, the n-channel cascode transistor 32 and thep-channel cascode transistor 42, respectively. In addition, r_(on1),r_(on2), r_(op3) and r_(op4) are output resistances of the amplificationtransistor 31, the n-channel cascode transistor 32, the p-channelcascode transistor 42 and the constant-current load transistor 41,respectively. Since the gain A of the cascode type inverting amplifieris several tens of times larger than the gain of the constant-currentload type source-grounded inverting amplifier shown in FIG. 3, Cinrepresented by (3) and (4) can be set smaller. Accordingly, in thetwo-dimensional amplification type solid-state image pickup device ofFIG. 5, the charge-voltage conversion efficiency q can be made evenlarger, so that higher sensitivity can be achieved.

FIG. 8 shows another modification of the two-dimensional amplificationtype solid-state image pickup device of FIG. 1. It is noted that, alsoin FIG. 8, the same constituent members as those of FIG. 1 aredesignated by the same reference numerals, and their description isomitted.

In this two-dimensional amplification type solid-state image pickupdevice, terminals of the reset transistor 6 and the capacitor 7 otherthan and opposite to their terminals connected to the charge detectionnode 8 are connected to the vertical signal line 11 instead of theoutput terminal 30 of the amplification transistor 3. The capacitor 9and the boosting signal line C(n) in FIG. 1 are omitted.

Also in this two-dimensional amplification type solid-state image pickupdevice, not a constant bias but a boosting signal φC is applied to thegate of the p-channel transistor 4 of the load part 21 by the controlsection 90. Thus, the voltage VD of the vertical signal line 11 is madevariable.

FIG. 9 shows operation timing of the two-dimensional amplification typesolid-state image pickup device under the control by the control section90 by focusing on the pixel group X(n) and the switched capacitoramplification part 20C corresponding thereto.

Operations of the periods T1, T2 are the same as those described withreference to FIG. 2. In this case, the boosting signal φC applied to thep-channel transistor 4 is set to the bias level Bias_p so that aconstant-current load is obtained.

In the next T3, in synchronization with the gate driving signal φT(n,1), the boosting signal φC goes grounding level GND, making the verticalsignal line 11 and the VD terminal short-circuited. Thus, the verticalsignal line 11 and the control section 90 serve as the second boostingmeans so that the voltage VD of the vertical signal line 11 is raised to2VDD, and the potential of the charge detection node 8 is deepened bycapacitive coupling via the capacitor 9 (with its capacitance Cin).

Accordingly, charge transfer from the photodiode 1 to the chargedetection node 8 is accelerated.

It is noted that potential change of the charge detection node 8 isequivalent to the one that results from distributing a voltage increaseof the vertical signal line 11 by the capacitance Cin of the capacitor 7and the capacitance C_(FD) of the charge detection node 8. It is alsopermissible that the voltage increase of the vertical signal line 11 isincreased depending on the amount of potential change necessary for thecharge detection node 8.

The operation of the next period T4 is the same as that described withreference to FIG. 2.

In this two-dimensional amplification type solid-state image pickupdevice, the capacitor 9 and the boosting signal line C(n) may beomitted, in comparison with that of FIG. 1. Accordingly, the area of thephotodiode 1 can be set larger proportionally, so that highersensitivity can be achieved.

FIG. 10 shows still another embodiment modification of thetwo-dimensional amplification type solid-state image pickup device ofFIG. 1. It is noted that, also in FIG. 10, the same constituent membersas those of FIG. 1 are designated by the same reference numerals andtheir description is omitted.

In the example of FIG. 1, the pixel group X is composed of k (k≧2)pixels within each column. However, in the example of FIG. 10, a pixelgroup X_(D) is set so as to stretch over adjoining two columns (in thiscase, 1st column and (I+1)th column). For instance, the n-th (where n≧2)pixel group X_(D) (n) with respect to the column direction in the figurecontains k pixels 10 in the 1st column and k pixels 10-1 in theadjoining (I+1)th column.

Output terminals of the individual transfer transistors 2 of the kpixels 10 in the 1st column are connected in common to form a chargedetection node 8. Similarly, output terminals of the individual transfertransistors 2 of the k pixels 10-1 in the (I+1)th column are connectedin common to form a charge detection node 8-1. Those charge detectionnodes 8, 8-1 are connected to each other by a connection line 50,forming substantially one charge detection node.

Such a pixel group X_(D) that stretches over two columns is set inpluralities in the row and column directions, respectively. Then,switched capacitor amplification parts 20 are provided in correspondenceto the pixel groups X_(D), respectively.

Reference characters T(n, O1), T(n, O2), . . . , T(n, Ok) denote gatedriving signal lines for the transfer transistors 2 of the pixels 10-1of odd-numbered columns, respectively, and T(n, E1), T(n, E2), . . . ,T(n, Ek) denote gate driving signal lines for the transfer transistors 2of the pixels 10 of even-numbered columns. Gate driving signals φT(n,O1), φT(n, O2), . . . , φT(n, Ok) and φT(n, E1), φT(n, E2), . . . ,φT(n, Ek) are to be applied to those gate driving signal lines,respectively, by a control section 90.

FIG. 11 shows operation timing of the two-dimensional amplification typesolid-state image pickup device under the control by the control section90 by focusing on the pixel group X_(D) (n) and the switched capacitoramplification part 20 corresponding thereto.

In this example of FIG. 11, in one horizontal period (1 H period),operations of the periods T1 to T4 for the pixels 10 of even-numberedcolumns as well as operations of the periods T1 to T4 for the pixels10-1 of odd-numbered columns are performed in iteration of two times.The one-time operations of the periods T1 to T4 are the same as thosedescribed with reference to FIG. 2. Thus, signals can be read out foreach of the pixels 10, 10-1 in each pixel group X_(D).

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

1-12. (canceled)
 13. An amplification type solid-state image pickupdevice driving method for driving an amplification type solid-stateimage pickup device which comprises a plurality of pixels each having aphotoelectric conversion element, and a transfer transistor fortransferring signal charge of the photoelectric conversion element,wherein signals derived from the respective pixels are amplified andoutputted to a signal line common to the pixels, and wherein: aplurality of pixel groups each comprising a plurality of pixels are set;switched capacitor amplification parts are provided in correspondence tothe pixel groups, respectively; each of the switched capacitoramplification parts has a charge detection node to which outputterminals of the transfer transistors of a corresponding one of thepixel groups are connected in common, an amplification part to which asignal on the charge detection node is to be inputted, a resettransistor connected between the charge detection node and an outputterminal of the amplification part, a first capacitance elementconnected between the charge detection node and the output terminal ofthe amplification part, and a select transistor connected between theoutput terminal of the amplification part and the signal line, and; aload part common to the switched capacitor amplification parts isconnected between the signal line and power supply of a specifiedvoltage so as to be combined with the amplification parts of theswitched capacitor amplification parts to make up inverting amplifiers,respectively; the method comprising: a first step of turning on thereset transistor of the switched capacitor amplification part to performa reset operation; a second step of, after the first step, turning onthe select transistor and turning off the reset transistor to perform aread operation of only a reset level; a third step of, after the secondstep, turning on the transfer transistor of the pixel to perform chargetransfer from the pixel to the switched capacitor amplification part;and a fourth step of, after the third step, turning off the transfertransistor and turning on the select transistor to perform a readoperation of a signal level, wherein operations of the first step to thefourth step are iterated for each of the pixels in each of the pixelgroups.
 14. An amplification type solid-state image pickup devicedriving method for driving the amplification type solid-state imagepickup device which comprises a plurality of pixels each having aphotoelectric conversion element, and a transfer transistor fortransferring signal charge of the photoelectric conversion element,wherein signals derived from the respective pixels are amplified andoutputted to a signal line common to the pixels, and wherein: aplurality of pixel groups each comprising a plurality of pixels are set;switched capacitor amplification parts are provided in correspondence tothe pixel groups, respectively; each of the switched capacitoramplification parts has a charge detection node to which outputterminals of the transfer transistors of a corresponding one of thepixel groups are connected in common, an amplification part to which asignal on the charge detection node is to be inputted, a resettransistor connected between the charge detection node and an outputterminal of the amplification part, a first capacitance elementconnected between the charge detection node and the output terminal ofthe amplification part, and a select transistor connected between theoutput terminal of the amplification part and the signal line; a loadpart common to the switched capacitor amplification parts is connectedbetween the signal line and power supply of a specified voltage so as tobe combined with the amplification parts of the switched capacitoramplification parts to make up inverting amplifiers, respectively; and,a second capacitance element whose one terminal is connected to thecharge detection node; and first boosting means connected to the otherterminal of the second capacitance element and serving for deepeningpotential of the charge detection node by capacitive coupling via thesecond capacitance element, the method comprising: a first step ofturning on the reset transistor of the switched capacitor amplificationpart to perform a reset operation; a second step of, after the firststep, turning on the select transistor and turning off the resettransistor to perform a read operation of only a reset level; a thirdstep of, after the second step, turning on the transfer transistor ofthe pixel, in a state that potential of the charge detection node hasbeen deepened by the capacitive coupling via the second capacitanceelement by the first boosting means, to perform charge transfer from thepixel to the switched capacitor amplification part; and a fourth stepof, after the third step, turning off the transfer transistor andturning on the select transistor to perform a read operation of a signallevel, wherein operations of the first step to the fourth step areiterated for each of the pixels in each of the pixel groups, whereby thesignal is read out from each of the photoelectric conversion elements ofthe pixel groups.
 15. An amplification type solid-state image pickupdevice driving method for driving the amplification type solid-stateimage pickup device which comprises a plurality of pixels each having aphotoelectric conversion element, and a transfer transistor fortransferring signal charge of the photoelectric conversion element,wherein signals derived from the respective pixels are amplified andoutputted to a signal line common to the pixels, wherein: a plurality ofpixel groups each comprising a plurality of pixels are set; switchedcapacitor amplification parts are provided in correspondence to thepixel groups, respectively; each of the switched capacitor amplificationparts has a charge detection node to which output terminals of thetransfer transistors of a corresponding one of the pixel groups areconnected in common, an amplification part to which a signal on thecharge detection node is to be inputted, a reset transistor connectedbetween the charge detection node and an output terminal of theamplification part, a first capacitance element connected between thecharge detection node and the output terminal of the amplification part,and a select transistor connected between the output terminal of theamplification part and the signal line, and; a load part common to theswitched capacitor amplification parts is connected between the signalline and power supply of a specified voltage so as to be combined withthe amplification parts of the switched capacitor amplification parts tomake up inverting amplifiers, respectively; wherein terminals of thereset transistor and the first capacitance element included in theswitched capacitor amplification part other than and opposite to theirterminals connected to the charge detection node are connected to thesignal line instead of the output terminal of the amplification part,and wherein the amplification type solid-state image pickup devicefurther comprises a second boosting means connected to the signal lineand serving for deepening potential of the charge detection node bycapacitive coupling via the first capacitance element, the methodcomprising: a first step of turning on the reset transistor of theswitched capacitor amplification part to perform a reset operation; asecond step of, after the first step, turning on the select transistorand turning off the reset transistor to perform a read operation of onlya reset level; a third step of, after the second step, turning on thetransfer transistor of the pixel, in a state that potential of thecharge detection node has been deepened by the capacitive coupling viathe first capacitance element by the second boosting means, to performcharge transfer from the pixel to the switched capacitor amplificationpart; and a fourth step of, after the third step, turning off thetransfer transistor and turning on the select transistor to perform aread operation of a signal level, wherein operations of the first stepto the fourth step are iterated for each of the pixels in each of thepixel groups.